1. Field of the Invention and Related Art
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a multi-color electrophotographic image forming apparatus in which a plurality of images are transferred and superimposed onto the same transfer material. The invention also relates to a multi-color electrophotographic image forming apparatus in which a plurality of electrophotographic photosensitive members are juxtaposed, on which images are formed through an electrophotographic process, and on which different color developed images are provided; these developed images are transferred onto an image receptor such as an image transfer material, an intermediate transfer material and continuous rolled paper fed to the developed images by a transfer material conveying device, whereby a multi-color image is formed by the sequential image transfer of the different color images. Applications of the present invention are not limited to electrophotographic color copying machines, but extend to various types of color printers or the like. For the sake of simplicity of explanation, a laser beam printer using an electrophotographic process is provided in this specification. An image bearing member such as the photosensitive member described above may be in any form, for example, in the form of a belt or a drum, as long as it is movable along an endless path, but for simplicity, the image bearing member will be described hereinafter as being in the form of a drum.
2. Description of Pertinent Information
Several different laser beam printers using an electrophotographic process have been proposed. One typical example is shown in FIG. 1.
In FIG. 1, an exemplary full-color laser beam printer is shown as having three image forming units Pa, Pb and Pc, each of which contains a photosensitive drum 1, 2 or 3 exclusively for the respective unit. Around each of the photosensitive drums, an image forming process means is disposed, although only a laser beam exposure means 4, 5 or 6 is shown. The process means may include known charging means, exposure means, developing means, transfer means, cleaning means and other necessary elements which are used in the known electrophotographic process. Below the photosensitive drum 1, 2 or 3, there is provided a transfer material conveying means 10 including a rotatable member or an endless belt 13, which is trained around rollers 11 and 12 and which is rotatable in the direction of an arrow to penetrate image transfer stations of the image forming units Pa, Pb and Pc. The transfer material conveying means 10 functions to convey the transfer material P fed from an unshown feeding station in the direction of the arrow through the transfer stations 7, 8 and 9 of the image forming units Pa, Pb and Pc.
In this structure, the first image forming unit Pa first forms a yellow toner visualized image on the photosensitive drum 1, which is then transferred at the transfer station 7 onto a transfer material P conveyed by the transfer material conveying means. While the yellow toner image is being transferred onto the transfer material P, the second image forming unit Pb produces a magenta toner visualized image on the photosensitive drum 2. The magenta toner image is transferred onto the transfer material P which now carries the toner image transferred from the first image forming unit Pa, when it is introduced into the transfer station 8 of the second image forming unit Pb. Similarly, while the magenta toner image is transferred onto the transfer material P, the third image forming unit Pc produces a cyan toner visualized image on the photosensitive drum 3, which is transferred onto the transfer material P now having the images transferred from the first image forming unit Pa and the second image forming unit Pb, when it is introduced into the transfer station 9 of the third image forming unit Pc.
If it is necessary or desirable to add a black toner image formation, a fourth (not shown) image forming unit may be employed.
In any case, when the superimposed transfer of the three or four color toner images are completed on the transfer material P, the transfer material P is conveyed to an image fixing station (not shown), where the superimposed images are fixed so that a permanent full-color image is formed on the transfer material P.
After the image transfer in each of the photosensitive drums, the cleaning means (not shown) removes the residual toner from the photosensitive member so as to prepare it for the next latent image formation.
The full-color image forming apparatus of this type is advantageous in that the overall process speed can be increased since the image forming units are provided for the respective colors and in that the passage of the transfer material can be made straight, and therefore, it is usable for thick paper or transparent film or the like. However, this apparatus has the problem of misregistering the color images formed by the different image forming units.
Misregistration of the transferred three or four color images appears as color misregistration or a variation in color. One of the causes for this misregistration has been found to be that the start position of the image transfer on the transfer material is different among the photosensitive drums due to nonuniform rotational drive of the photosensitive drums. In order to solve this problem, it has been proposed to couple a rotary encoder to the photosensitive drum, thus, accurately controlling the rotation of each of the photosensitive drum. This method has turned out to be able to provide good results, but the structure implementing this method and the control system are complicated, which leads to an increase in the cost.
With the development of dot matrix or raster line type image writing systems, for example, not only with multi-color laser beam printers but also with LED printers, and liquid crystal shutter printers or the like, the tolerance of the non-uniform speed of the image bearing member attributable to non-uniform rotational drive to the image bearing member, typically a photosensitive drum or the like, becomes very severe. For example, in the case of raster line system for producing an image at a density of 16 picture elements per mm, a correct pitch is 0.0625 mm, and an error of this pitch is seen as a non-uniform density by human eyes.